In the early days of satellite communications, there were few downlink earth stations and those in existence were essentially large antenna dishes operative with wired communications hubs. Any signals received at these large earth stations were distributed through wires and cables to numerous destinations and even other communications hubs. Thus, many earth stations were positioned in metropolitan areas and acted as communications hubs, and distributed communication signals often in a broadcast fashion to other hubs, regional centers, or local sites via cable. It was not convenient to have a large number of smaller, earth station terminals.
This scenario changed with the advent of very small aperture terminal (VSAT) communications systems or networks. These VSAT systems are cost-effective communications networks that allow many smaller VSAT terminals to be geographically dispersed and located in many different areas, including rural and metropolitan areas. VSAT networks support internet, voice/fax, data, LAN and many other communication formats.
A VSAT network usually includes a large central earth station known as a central hub (or master earth station), a satellite transponder, and a large number of geographically disbursed, remote VSATs. The satellites are positioned in a geostationary orbit about 36,000 kilometers above the earth. A VSAT terminal receives and transmits signals via the satellite to other VSATs in the network. The term “very small” used in the VSAT name refers to the small antenna dish that typically is about 3 to about 6 feet in diameter and could be mounted in almost any location, such as a roof, building wall, or on the ground. The VSAT terminal has an outdoor unit (ODU) that includes an antenna, low noise blocker (LSB) in some instances, and a VSAT transceiver as part of the outdoor electronics and other components. The antenna usually includes an antenna reflector, feed horn and an antenna mount or frame. The outdoor electronics constitute part of the outdoor unit and include low noise amplifiers (LNA) and other transceiver components, such as a millimeter wave (MMW) transceiver.
The indoor unit (IDU) can be an interface, such as a desktop box or PC, that contains the electronics for interfacing and communicating with existing in-house equipment such as local area networks, servers, PCs and other equipment. The indoor unit is usually connected to the outdoor unit with a pair of cables, e.g., coaxial cable. Indoor units also include basic demodulators and modulators.
In the next few years a number of Ka-band (27.5 to 30 GHz) satellites will be launched that will enable remote Internet access via 2-way communication with user terminals. To successfully compete with other Internet services such as Digital Subscriber Line (DSL) and cable modem, the cost of very small aperture terminals (VSATs) must be reduced to a low level. As noted before, each very small aperture terminal tyipcally includes an antenna, a diplexer, and a millimeter wave (MMW) transceiver. In many current VSAT designs, the MMW transceiver circuit accounts for almost 75% of the total cost of a VSAT terminal. Unlike the lower frequency Ku-band transceivers, which can be built from low cost discrete components using low cost soft board, such as Rogers board, a Ka-band transceiver requires much tighter tolerances because of its inherent shorter wavelength. One current method pursued by many manufacturers is to pre-package the Ka-band MMIC chips in surface mount packages using traditional surface mount technology (SMT) assembly methods. Although this method is widely used, it has not been very successful at driving down the costs of VSATs because the packaging of MMIC's and the required tuning after assembly has been expensive.
In addition to the cost issue, as the number of VSAT terminals increases (to perhaps millions of units in the next few years), the amount of power transmitted from the ground unit (VSAT terminal) to the satellite transponders will have to be better controlled. Most VSAT terminals require low power to operate in clear weather. High power is only required to overcome weather and maintain a high rate of service availability. Continuously “blasting”, i.e., transmitting high power signals, will also reduce the transceiver reliability as maximum heat is constantly generated, shortening component life.